1. Field of the Invention
This invention relates to a piston stem insert for a carpule based piston by which a piston stem can be easily and reliably attached to or detached from the piston without damaging the piston or affecting its ability to be moved through the carpule.
2. Background Art
It is known within the relevant art to locate a piston stem insert within a piston body so that a piston stem can be attached to (or removed from) the piston for controlling the movement of the piston through a fluid filled cylinder, such as a medication carpule. However, there are certain unresolved problems that are associated with the interconnection of a conventional piston stem insert and a piston stem Which have resulted in damage to the piston and/or prevented an easy detachment and removal of the piston stem from the piston.
More particularly, a piston stem which is screwed down too tightly against its piston stem insert is known to drive the insert inwardly through the piston. As a result, the piston might swell, crack or be subjected to some form of damage. The frictional force generated between the piston and the carpule through which said piston is to slide will be increased as the size of the piston increases. Consequently, a greater force will be required to drive an expanded or swelled piston through its carpule for explusing the fluid contents of such carpule. Moreover, the piston stem can be so tightly connected to the piston stem insert that disconnecting the stem from the insert becomes undesirably difficult. That is, the combination piston stem and insert may rotate freely relative to a piston that is split or cracked. Thus, there may be no easy way to rotate the piston stem out of engagement from its piston stem insert when it is desirable to remove the stem for disposal.
Yet another problem which has been experienced by conventional carpule based pistons occurs as the carpule, in which the piston and piston stem insert are carried, is subjected to increased atmospheric pressure, such as at a high elevation. The atmospheric pressure at such high elevation has been known to prematurely and inadvertently displace the piston through the carpule. In some cases, the piston is pushed proximally and completely out of the carpule while the carpule is still in its as-packaged configuration. The unintended spillage of fluid from a carpule whose piston has been prematurely removed would render the carpule unusable and subject to disposal and waste.